In general, if the "r" is placed in the end of the word or in the middle of a word, where the syllable is not the strongest (a sílaba atôna), it will be pronounced as the "r" in "door" /r/. For example: porta, Brasil.
If the word has two "rr" its sound will be as in "hand" /h/.
It's important to remember, though, that there are a lot of accents around the Portuguese speaking countries. In some places in Brazil people pronounce the "r" very strong as in "ruler", in some other places the "r" has a little vibration like in the sound a motorbike engine makes :) (it's hard to describe it)
I hope I could help!